A voltage controlled oscillator is widely used as means for generating a local oscillation signal for a wireless communication device. When the voltage controlled oscillator is manufactured as a radio frequency IC, it is necessary to increase an oscillation frequency range thereof, in order to accommodate variations among components thereof, which occur in a semiconductor manufacturing process. Moreover, in recent years, a demand arises that an oscillation frequency of a voltage controlled oscillator be adjustable in a wide frequency range so as to adapt the voltage controlled oscillator to communication systems using different frequency bands.
FIG. 13 is a diagram showing an exemplary configuration of a conventional voltage controlled oscillator 1d having an increased oscillation frequency range (for example, Patent Document 1). In FIG. 13, the conventional voltage controlled oscillator 1d includes: an inductor circuit having inductors 3; a first variable capacitance circuit formed with variable capacitance elements 4; a second variable capacitance circuit formed with variable capacitance elements 5; a third variable capacitance circuit formed with variable capacitance elements 6; a negative resistance circuit formed with transistors 9; a bias circuit 16; and switches 54 and 55. The inductor circuit, the first to third variable capacitance circuits, and the negative resistance circuit are connected in parallel with each other, and thus form an oscillation circuit.
In the conventional voltage controlled oscillator 1d, the switch 54 or 55 switches the direction of connection of a capacitance value control terminal of at least one of the two variable capacitance elements 5 and 6 which are provided in parallel with each other. This can cover different oscillation frequency ranges in accordance with the switched direction of connection. Thus, a plurality of kinds of oscillation frequency characteristics, having a lowered frequency sensitivity, can be obtained. The frequency sensitivity indicates a rate of variation of the oscillation frequency with respect to a frequency control voltage.
Patent Document 1: Japanese Laid-Open Patent Publication No. 2007-104152